Understanding an Electrocardiogram (ECG)

An electrocardiogram (ECG) measures electrical changes in the body. These changes occur mainly in
muscles and nerves when they become active. The heart, being a muscle with well-organized
electrical activity, makes it easy to detect its electrical signals from outside the body.

How the ECG Machine Works

To record the heart's electrical activity, the ECG machine uses two electrodes placed on the skin to
measure the voltage between them over time. The machine has a roll of paper that moves at a
steady speed, and a stylus that moves up and down according to the voltage changes, creating a
visual representation of the heart's electrical events.

 Stylus Movement: The stylus moves up for positive voltage and down for negative voltage,
drawing a graph of voltage over time on the moving paper.

Since electrical activity can travel in different directions, we need multiple sets of electrodes to get a
complete picture. Electrical changes are only detected when the electrical wave is moving toward or
away from an electrode. If it's moving at a right angle, it won’t be picked up.

The Standard Leads

By attaching electrodes to the left arm, right arm, and either leg, we create three "standard limb
leads" called I, II, and III. Each lead measures voltage between two points:

 Lead I: Left arm vs. right arm. The left arm is the positive electrode. If electrical activity
moves toward the left arm, the stylus moves up. This lead is best for detecting horizontal
electrical activity.

 Lead II: Right arm vs. leg. It is best for seeing electrical activity moving downward and to the
left.

 Lead III: Left arm vs. leg. It measures electrical activity moving downward and to the right.

The Augmented Leads

In addition to the standard leads, three more leads can be created using combinations of electrodes.
These are called augmented leads (aVR, aVL, aVF). For these leads, two electrodes are connected to
form an average, which is then connected to the remaining electrode, allowing for more detailed
readings of the heart's electrical activity.

ECG Leads Overview

1. Limb Leads:

o Lead F: Positive at the foot, best for detecting electricity moving vertically. Positioned
between Lead II and Lead III.

o Lead L: Compares right arm and foot, with the left arm as positive. It detects leftward
and slightly upward electrical activity. Positioned between Lead I and Lead III.

o Lead R: Compares left arm and foot with the right arm as positive. It detects
rightward and slightly upward activity, often resulting in inverted waveforms
compared to other leads.

Limb Lead Directions

  Each lead has a specific angle:

o Lead I: 0 degrees

o Lead L: -30 degrees

o Lead II: +60 degrees

o Lead F: +90 degrees

o Lead R: +30 degrees

Limb Lead Vector Diagram

 This diagram visually represents the orientation and directionality of each lead. Each arrow
shows the positive direction for detecting electrical activity, crucial for identifying potential
pathologies.

Chest Leads (V Leads)

 The six chest leads (V1 to V6) sample electrical activity from different parts of the heart,
providing a more detailed view than limb leads. They detect front-to-back and side-to-side
electrical movements, important for diagnosing conditions like ventricular hypertrophy.

ECG Grid

 The ECG paper is marked with millimeter lines:

o Time Measurement: 1 mm = 0.04 seconds; every 5 mm = 0.2 seconds; 25

mm/second movement means 1 second equals 5 large boxes.

o Voltage Measurement: Each vertical millimeter represents 0.1 millivolt; 10 mm = 1

millivolt.

Machine Operation

 Modes: The ECG machine can operate in automatic or manual mode, allowing for lead
selection and recording.

 Specifications: The device uses standard 12 leads with specific color-coded connections for
easy identification.

Usage

 Understanding these leads and their orientations helps in diagnosing heart conditions, such
as conduction blocks or myocardial infarctions, based on the waveforms observed in the
ECG.

Manual Operation Instructions

1. Switching Modes:

o Press the AUTO/MANUAL key to switch to manual mode. The red AUTO light will
turn off.

2. Selecting Leads:

o Use the lead selection keys to choose which lead you want to record.
 3. Starting the Recording:

o Press the START/STOP button to begin recording.

4. Testing Signal:

o Press the Imv key to record a test signal.

5. Stopping the Recording:

o Press the START/STOP button again to stop the recording.

6. Repeating the Process:

o You can repeat these steps for any other lead you want to record.

Specifications

 Leads: The machine uses a standard set of 12 leads.

o Right Arm (RA): Red tip at the black cord end

o Left Arm (LA): Yellow tip at the black cord end

o Left Foot (LF): Green tip at the black cord end

o Right Foot (RF): Black tip at the black cord end

o Chest Leads (V1 to V6): These are used for monitoring heart activity.

 Input Mode:

o Patient Leakage Current: Should be 10 mA or less to ensure safety.

o Cardiac Pulse Pacing Suppression: This feature helps to record clear ECG data even if
there is a cardiac pulse present.

o Test Signal: A test signal called ImV can be recorded.

o Test Duration: Each test lasts for 3.2 seconds.

 Frequency Response: The machine can measure frequencies from 0.05 Hz to 100 Hz.

 Common Mode Rejection Ratio (CMRR): The machine has a CMRR of 100 dB, which helps
reduce noise in the ECG readings.

Automatic Operation Instructions

1. Power On:

o When you turn on the machine, it should show AUTO on the display, and a red light
on the key will light up, indicating that it’s in automatic mode.

2. Lead Display:

o The machine will automatically display the ECG readings from the 12 leads: I, II, III,
aVR, aVL, aVF, V1, V2, V3, V4, V5, and V6 one after another.

3. Recording:
 o You can obtain a record of these readings using the stylus and paper.

4. Adjustments:

o You can adjust the paper speed and sensitivity to suit your needs.

5. Filters:

o You can also select different filters as required for clearer readings.

Maintenance Procedure

1. Power On:

o Turn on the machine and let it warm up for about a minute.

2. Set Function:

o Set the function switch to Run and the lead selector switch to STD. Check if a trace
appears on the screen.

3. Imv Calibration:

o Press the Imv CAL button several times and check:

 Pulse Edges: Are the edges of the pulse changing?

 Sensitivity Adjustment: Can you adjust it to get at least 10 mm of upward

movement on the trace?

 Pulse Shape: Does the pulse look reasonably square?

4. Position Control:

o Move the position control through its entire range. Ensure the stylus moves to the
edges of the paper without sloping at the top or bottom.

5. Check Connections:

o Connect all electrode connectors together at the patient end of the cable. Then,
switch through all 12 lead positions. You should see a stable baseline on the paper in
every position. If any lead isn’t stable, note which one to find the broken wire in the
cable.

6. Sensitivity Adjustment:

o Set the sensitivity so that it shows at least 10 mm of movement when the lead
selector is on STD and the 1 mV CAL button is pressed.

7. Imv Test:
 o Press and hold the Imv CAL button. The stylus should move up by 10 mm and then
slowly return to its original position. The return should take longer than 7 mm over
16 large divisions. This checks the machine's low-frequency response.

ECG Faults and Troubleshooting

General Note: Internal faults in ECG machines are rare, but many malfunctions are due to operator
error and can often be fixed easily.

Example 1

Symptom:
The machine is running, but the thermal stylus does not write or writes very lightly.

Possible Causes:

1. The stylus tip is not hot enough.

2. The stylus is not pressing down hard enough on the paper.

Troubleshooting Steps (while the machine is running):

1. Take an insulated tool, like a screwdriver, and gently press the stylus onto the paper.

2. Check the paper:

o If a dark line appears, the issue is too little pressure.

o If no dark line appears, the problem is that the stylus is not hot enough.

Solutions

1. For No Heat:

o Check the heater voltage at the stylus wires.

o If the voltage is correct, replace the stylus.

o If the voltage is not correct, check the service manual for further troubleshooting
steps.

2. Adjust Stylus Pressure:

o Don’t guess the right pressure, as it can vary by model (typically between 2 and 20
grams).

o Use a stylus pressure gauge to measure the pressure.

o Refer to the manufacturer’s service manual for the correct pressure value.

o Some models may require the pressure to be set at a specific heater voltage.

Example 3

Symptom:
Poor recording on the ECG.
Possible Causes:

 Electronic or mechanical issues.

 Faulty lead switch or input connector.

 Bad patient cable or improper connection to the patient.

Troubleshooting Steps:

1. Set the lead selector switch to STD. Connect all electrodes together and press the Imv CAL
button.

2. If a normal calibration pulse appears, the issue is likely with the connection to the patient.

3. If the problem continues, try using a patient cable that you know is working or use a dead
head plug (an ECG connector with all pins connected together).

o If the problem goes away, replace the faulty patient cable.

o If the problem remains, the issue is likely inside the machine.

Artifacts in ECG Recordings

ECG recordings need to be done in challenging environments, where interference can occur, such as:

1. 60 Hz Interference:

o This interference often comes from nearby power lines and can affect the electrodes
and wires.

o Normally, the ECG pre-amplifier can filter out this interference by creating a
differential signal.

o However, issues arise if there’s a lack of electrode jelly or if the electrodes aren’t
making good contact, especially on moist or sweaty skin.

o A loose or broken ground connection on the ECG machine can also cause this
interference, as can certain power supply problems that introduce a 120 Hz ripple.

Troubleshooting Steps:

o Short all electrodes together and check each lead position:

1. If the interference stops, the issue is likely due to a bad electrode, lack of
jelly, or poor skin preparation.

2. If interference continues in all positions, the problem is likely internal to the

machine.

3. If the issue occurs only in certain leads, check for an open wire in the patient
cable. Use an ohm meter to identify which electrode is causing the problem.

2. Muscle Jitter (Somatic Tremor):

o This is caused by muscle activity and appears irregular in both size and frequency.
 o It can sometimes be reduced by having the patient lie still. However, if the patient
has tremors or needs long-term monitoring, electronic filtering may be necessary.

o Most bedside monitors are designed to respond only to frequencies between 30 to

50 Hz to minimize the impact of muscle jitter.

Wandering Baseline

The wandering baseline is usually caused by the electrode moving on the skin. While electronic
issues can sometimes lead to this problem, they are rare. Most often, it happens due to changes in
how well the electrode is making contact, which affects the signal.

Common Causes of Wandering Baseline:

1. Loose or poorly filled electrodes: If the electrodes aren’t secure or properly applied, they
can cause the baseline to wander.

2. Dangling patient cable: If the patient cable is hanging down, it can pull on the electrodes and
create tension.

3. Movement with respiration: The electrodes or cables can shift as the patient breathes.

Solutions:

 Make sure the electrodes are secure to fix the first issue.

 Properly dress and secure the patient cable to resolve the other problems